Poorly conducting oxides must first be preheated. For this purpose, according to U.S. Pat. No. 4,224,100 pieces of a metal which corresponds to that of the oxide to be melted are used and are embedded in the powdered oxide. On the basis of the induced eddy currents the applied electromagnetic field first heats the pieces of metal which in turn melt the oxide powder in the immediate proximity. In the forming melt the field of the high frequency coil can directly couple-in due to the higher electric conductivity of the melt. By increasing the high frequency power further oxide powder is continuously melted until melt has formed in the proximity of the crucible wall. A water-cooled inner crucible surface ensures that between it and the hot melt a densely sintered specific layer which is in a solid state is formed which protects the crucible from attack by the melt. The melt used for the preheating is converted into the oxide to be melted by oxygen from the air.
It has now been found that perfect crystals cannot be grown by means of this known crucible. By slowly cooling the melt, single crystals, up to a few centimeters large, can grow spontaneously on the bottom and wall of the crucible and hinder each other during the growth. The results are considerable mechanical stresses and other crystal defects.